Provided are a projection type display device and an operation assistance method that enable an operator to enjoy an operation assistance function even in the case of occurrence of abnormality in an optical modulation unit or an image information input unit during operation of a vehicle. A system control unit 47 of a HUD, if sensing abnormality in an optical modulation device 44 and a drive unit 45, stops a light source unit 40 and, furthermore, in the case of image information generated by a projected image information generation unit 52 including image information that calls attention of the operator to the front field of view, displays an alert image based on the image information on a liquid crystal display device 61 in a meter cluster.